Generally, compound-type superconducting wires are produced by a method which comprises the steps of making a wire of metallic materials which form a superconductor upon reaction with each other, and heat-treating said wire to form said superconductor.
If said superconductor is in the form of, e.g., Nb.sub.3 Sn, a composite wire comprising bronze, which is a Cu-Sn alloy, and Nb wires enveloped in said bronze, is produced and then heat-treated to allow the Sn in the bronze and the Nb in the wire to react with each other such that 3Nb+Sn .fwdarw.Nb.sub.3 Sn, thereby forming a continuous Nb.sub.3 Sn structure in the Cu matrix (bronze process); alternatively, a wire of Cu-Nb alloy is plated with Sn and then heat-treated to produce said reaction (in-situ process). Said composite wire may also be produced by mixing Cu powder with Nb powder, sintering the same, drawing the sintered mass into a wire; the latter is then plated with Sn and heat-treated (powder process). In each of these methods, however, voids, called Kirkendall voids, are inevitably produced in the wire because of a change in the atomic arrangement due to the reaction. The superconducting compound produced in the wire is not fit to be rolled, drawn or otherwise worked. Therefore, the present state is such that after said heat treatment, the product, still containing voids, is used as a superconducting wire without being subjected to any treatment for removal of the voids.
It is well known that the presence of such voids in the superconducting wire lowers the heat conductivity of the wire and is liable to cause damage to the layer of said compound when the wire is stressed, detracting from the electrical properties of the compound-type superconducting wire.
Superconducting compounds are so brittle as not to allow the use of conventional void eliminating means, such as rolling and drawing, as described above. Thus, the presence of such voids in compound-type superconducting wires is a serious problem, and there is an urgent need to develop an effective void eliminating means. In addition, IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-15, NO. 1, January 1979 carries an article entitled "KIRKENDALL VOIDS--A DETRIMENT TO Nb.sub.3 Sn SUPERCONDUCTORS", pp. 178-181, which discloses means for suppressing the formation of Kirkendall voids. ACTA METALLURGICAL, VOL. 23, 1975 carries an article entitled "PRESSURE SINTERING BY POWDER LAW CREEP", pp. 1277-1285, which discloses a mathematical analysis of voids and of sintering.